Modern internal combustion engines such as automotive engines, particularly diesel engines, have high efficiency and have approached the optimal Carnot efficiency, which describes the maximum theoretical efficiency of converting heat energy into mechanical energy. Future increases in efficiency are only to be expected in small quantities and are limited by the used materials and the ever-increasing emission standards. One possibility for the remaining increase in efficiency is the utilization of waste heat of an internal combustion engine, in particular the use of the waste heat from a vehicle engine, since for example in the case of a diesel engine the exhaust gas contains approximately 60% of the calorific value of combustion and in the case of a gasoline engine between 70% and 80% of the calorific value of combustion.
The use of waste heat to increase the efficiency of steam turbine is known, for example in a natural gas powered combined cycle power plant or in the waste gas turbines of large marine engines. The energy contained in the exhaust gas stream is used to heat a secondary heat engine. However, these secondary heat machines are very complex and cannot be used in a practical manner in vehicles due to the required size and the required weight of the heat exchangers for the evaporator and the cooler.
In internal combustion engines, in particular vehicle engines, it is also to be noted that only about 50% of the waste heat calorific energy escapes through the exhaust pipe and the remaining approximately 50% of waste heat is conveyed via the cooling power of the radiator in currently unusable temperatures of about 120° C. to 140° C.